1. Field of the Invention
The present invention relates to a process for growing silicon single crystal and a process for producing a silicon wafer, in particularly, to a process for growing silicon single crystal which is capable of pulling it up at a pulling rate at which a silicon single crystal including a laser scattering tomography defect-generating region is grown, without generating a hydrogen defect.
Priority is claimed on Japanese Patent Application No. 20005-208525, filed Jul. 19, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
As a production method for a silicon single crystal which is a material of a silicon wafer, a growing method by the Czochralski method (referred to as “CZ method” hereinafter) is known.
It is known that minute defects will be formed during the production process of a device, i.e. a grown-in defect is generated in the silicon single crystal produced by the CZ method. FIG. 1 is a cross-sectional view for explaining the distribution state of defects in a radial direction of the silicon single crystal obtained by the CZ method. As shown in FIG. 1, the grown-in defect of the silicon single crystal obtained by the CZ method consists of hole defect having a diameter of approximately 0.1 to 0.2 μm, which is called a laser scattering tomography defect or COP (crystal-originated particle), etc. and a minute dislocation having a diameter of approximately 10 μm, which is called a dislocation cluster.
Moreover, as for the silicon single crystal shown in FIG. 1, an oxygen-induced stacking defect (referred to as “OSF” hereinafter) has appeared in the shape of a ring at the region of approximately ⅔ of the outer diameter. At the inner portion of the OSF-generating region where the OSF will be generated, there is a region (a laser scattering tomography defect-generating region) where approximately 105 to 106/cm3 of a laser scattering tomography defect is detected, whereas at the outer portion there is a region (a dislocation cluster-generating region) where approximately 103 to 104/cm3 of a laser scattering tomography defect is present.
FIG. 2 is a figure for explaining the distribution state of defects of the cross-section of a silicon single crystal which is produced by reducing gradually the pulling rate at the time of pulling during growth. It should be noted that FIG. 1 is a sectional view of the silicon single crystal grown at the pulling rate corresponding to the position of A shown in FIG. 2.
As shown in FIG. 2, at the stage where the pulling rate is high, a ring-like OSF-generating region appears in a crystal circumference part, and the inner portion of the OSF-generating region serves as a laser scattering tomography defect-generating region where many laser scattering tomography defect will be generated. And the diameter of the OSF-generating region decreases gradually and the dislocation cluster-generating region where dislocation clusters will be generated appears at the outer portion of the OSF-generating region, and then the OSF-generating region disappears and the dislocation cluster region will appear in the whole surface.
Moreover, an oxygen precipitation-promoted region (PV region) where an oxygen precipitate (BMD: Bulk Micro Defect) can be formed exists on the outside which is adjacent to a ring-like OSF-generating region, and there is an oxygen precipitation-inhibited region (PI region), where no oxygen precipitate is generated, between the oxygen precipitation-promoted region and the dislocation cluster-generating region.
The silicon single crystal in which a laser scattering tomography defect is detected has a negative influence which is smaller than that of a silicon single crystal from which a dislocation cluster is detected, and excels in productivity because it is possible to increase the pulling rate. However, as integrated circuits have been down-sized in recent years, deterioration of gate oxide integrity due to a laser scattering tomography defect has been pointed out.
Moreover, as a hot-zone structure in the case of growing a silicon single crystal by the CZ method, for example, a hot-zone structure in which the temperature gradient (Gc) at a central portion of the crystal is the same as or greater than the temperature gradient (Ge) at a perimeter of the crystal (Gc≧Ge) has been proposed in, for example, Patent document 1.
FIG. 3 is a drawing for explaining the distribution state of defects of a cross-section of a silicon single crystal which is grown while gradually decreasing the pulling rate at the time of pulling, with a growing apparatus having a hot-zone structure in which the temperature gradient (Gc) at a central portion of the crystal is the same as or greater than the temperature gradient (Ge) at a perimeter of the crystal (Gc≧Ge).
As shown in FIG. 3, if it grows at a pulling rate ranging from B to C shown in FIG. 3, with a growing apparatus which has the hot-zone structure in which the formula of (Gc≧Ge) is satisfied, then the temperature gradient G on the side of a crystal at near the solid-liquid interface will be controlled, so that a silicon single crystal which serves as a uniform defect-free region over the whole wafer surface is obtained.
Furthermore, in Patent document 1, a technology for increasing the pulling rate margin of a defect-free crystal by adding hydrogen to a pulling furnace using a growing apparatus having a hot-zone structure in which the formula of (Gc≧Ge) is satisfied has been proposed. FIG. 4 is a drawing for explaining the distribution state of defects of a cross-section of a silicon single crystal which is grown while decreasing gradually the pulling rate at the time of pulling, and supplying an inert gas in which hydrogen is added to the pulling furnace, using a growing apparatus having the same hot-zone structure as in FIG. 3 in which the formula of (Gc≧Ge) is satisfied.
If, as an atmospheric gas for growing a single crystal, a mixed gas consisting of an inert gas and hydrogen is used, then the pulling rate at which an OSF-generating region disappears in the central part of a crystal will increase. Therefore, as shown in FIG. 4, it is possible to make a critical rate of a range of pulling rate (the range of B to C in FIG. 3, and the range of D to E in FIG. 4) at which a defect-free crystal can be pulled up higher than that of the example shown in FIG. 3 in which no hydrogen gas is added to the pulling furnace.    [Patent Document 1] International publication WO 2004/No. 083496 pamphlet
In the technology disclosed in Patent document 1, it is possible to suppress the generation of COP, which is a laser scattering tomography defect, by adding hydrogen into the pulling furnace, without decreasing the pulling rate to be not higher than the critical pulling rate at which an OSF-generating region will be generated, however, in the case of growing a silicon single crystal at a pulling rate which is not lower than the critical pulling rate at which an OSF-generating region will be generated, a large cavity consisting of a hydrogen defect will be generated, which has been a problem. The hydrogen defect will not disappear even if it is subjected to a heat treatment, and hence it is impossible to use a silicon single crystal in which hydrogen defects exist as a silicon wafer for use in a semiconductor.
The present invention is made in view of the above circumstances. That is, it is an object of the present invention to provide a process for growing a silicon single crystal which is capable of growing a silicon single crystal at a pulling rate which is not lower than the critical pulling rate at which an OSF-generating region will be generated even if an atmospheric gas for growing a single crystal contains a gaseous hydrogen-containing substance, and of growing a silicon single crystal which contains an OSF-generating region and no hydrogen defects.
Moreover, it is another object of the present invention to provide a process for producing a hydrogen defect-free silicon wafer which is extracted from a straight trunk portion of a silicon single crystal which has been grown by the process for growing a silicon single crystal in the above.